More OOP: Objects and Constructors Here, we will continue our discussion of objects. Suppose that you have a class with multiple attributes. As a simple example, imagine a class called Point3D that holds three public variables: x, y, and z, which are the coordinates of a point in space. Suppose that you want to create a point, and set its coordinates to (3.0, 4.0, 5.0). You could do it as follows: Point3D p = new Point3D(); p.x = 3.0; p.y = 4.0; p.z = 5.0; However, this takes 4 lines of code. Instead of going through that sequence, it is possible to write a special method called a constructor that will initialize the Point3D with three values when it is created. A constructor is simply a method that (1) has no return value, and (2) has the same name as the class. For example, a constructor with one parameter for Point3D that sets all three coordinates to the same value could be written: public Point3D(double a) { x = a; y = a; z = a; } You cannot call a constructor directly. Instead, it gets called when you create an object using the keyword new. For example, if you make a constructor with three parameters for Point3D, you can write the following code to initialize several points: Point3D p = new Point3D(3.0, 4.0, 5.0); Point3D q = new Point3D(6.0, -7.0, 8.2);

Each one of these replaces 4 lines of code, making it easier to type and read. Part 10)

Write the class Point3D in a file called Point3D.java. Use the following directions: a) Give the class 3 public double variables: x, y, and z. b) Write a public constructor that takes 3 double variables: xx, yy, and zz. (I'm not naming them x, y, and z so as not to conflict with the instance variables.) c) Make sure that your file compiles. (Note that it will not run since it does not have a main method.)